The present invention relates to hydrodynamic transmissions, especially for use in vehicles, and more particularly to improvements in transmissions of the type having one or more hydrodynamic torque converters in the forward as well as in the reverse drive train. More particularly, the invention relates to improvements in transmissions of the type disclosed in commonly owned U.S. Pat. Nos. 3,566,715 to Keller et al. and 3,677,004 to Muller et al.
Transmissions of the class disclosed in the aforementioned patents are often utilized in locomotives which are intended for shunting or switching service in marshalling yards, in earthmoving vehicles and in similar conveyances which are designed to perform operations involving frequent reversals in the direction of travel. An advantage of such transmissions is that the shifting into different speeds and/or changing the direction of movement does not cause any wear upon the component parts. This is due to the fact that such operations merely involve emptying or filling the working circuits of selected converters.
An earthmoving apparatus, e.g., a scraper, is often operated under circumstances which necessitate repeated back-and-forth movements during loading of the bowl. Such movements are normally performed at low speeds. At the same time, traction force or output torque must be sufficiently high to insure that the scraper is not arrested when the bowl encounters a pronounced resistance to forward movement at a speed in the range of 2 to 4 miles per hour. In addition, the transmission should be capable of insuring rapid movement of the vehicle during long-distance hauling of loads, e.g., when the vehicle is driven on paved roads.
It has been found that heretofore known hydrodynamic transmissions are not entirely satisfactory for use in earthmoving vehicles, especially in scrapers, bulldozers and analogous apparatus, even though the required performance characteristics of a hydrodynamic transmission in a locomotive do not deviate appreciably from those which are expected from a hydrodynamic transmission in a heavy-duty earthmoving apparatus. This is due to the fact that the circumstances under which an earthmoving vehicle is operated are often basically different from those under which a locomotive for switching or shunting is put to use. For example, a very high percentage of the power output of the prime mover in an earthmoving vehicle is used up for operation of auxiliary equipment, especially the hydrostatic pump or pumps of the lifting mechanism for the bowl.
Conventional hydrodynamic transmissions for earthmoving apparatus (e.g., those disclosed in U.S. Pat. No. 3,347,113 to Ramsel) employ a hydrodynamic torque converter and a range transmission. The transmission of Ramsel can furnish converter or straight mechanical drive and can change speeds while under load. However, when shifting into a different gear, or during frequent changes in the direction of movement, the friction clutches of the patented transmission are subjected to extensive wear and the changes in speed take place without gradual transition from higher speed to lower speed or vice versa.